A conventional method of suppressing cogging torque in this type of dual-rotor motors is disclosed in a prior art of Patent Literature 1. More specifically, a conventional dual-rotor motor has inner teeth and outer teen on a stator. In general, an inner rotor is provided facing the inner teeth, and an outer rotor is provided facing the outer teeth.
In this structure, the center of inner slot open, the center of outer slot open, and a rotating center are generally aligned in the same straight line. In addition, an inner slot angle and an outer slot angle are often made equivalent. The inner slot open refers to distance between heads of adjacent inner teeth, and the outer slot open refers to distance between heads of adjacent outer teeth. The inner slot angle refers to an angle formed by straight lines connecting the rotating center and both ends of the inner slot open. The outer slot angle refers to an angle formed by straight lines connecting the rotating center and both ends of the outer slot open.
In this structure, cogging torques inside and outside the stator have the same phase. Accordingly, combined cogging torque becomes larger than individual inner and outer cogging torques.
Therefore, for example, Patent Literature 1 proposes a technology of suppressing combined cogging torque by changing inner and outer slot open positions and widths so as to adjust phases and amplitudes of inner and outer cogging torques.
On the other hand, a range of methods of suppressing cogging torque are also proposed for single-rotor motors. One conventional method of suppressing cogging torque is, for example, a prior art disclosed in Patent Literature 2. More specifically, an arc portion is provided at the center and a flat and sloped portion is provided at the left and right of a tip head of teeth provided on the stator. These flat and sloped portions at the left and right are formed such that a gap between a teeth head and the rotor broadens toward both ends. Provision of this flat and sloped portion improves cogging torque by suppressing amplitude of cogging torque.
However, the conventional dual-rotor motor of Patent Literature 1, which sets the position and width of the inner and outer slot open, has a disadvantage in packing factor and work efficiency. If positions and widths of both slot opens are set as in this conventional method, appropriate combined cogging torque is achieved, but the inner and outer slot open positions and widths become uneven. This unevenness reduces the packing factor of windings around the stator, and a complicated shape decreases the work efficiency in the winding operation.
More specifically, if the inner and outer slot opens are provided off the rotating center, the shape becomes complicated for the winding operation. Accordingly, the work efficiency decreases, and an expensive winder for complicated winding process may become necessary. Even if the inner and outer slot open positions are aligned relative to the rotating center, different slot open widths lead to lower winding packing factor because the winding width needs to conform to a narrower slot open width.
If only an amplitude of cogging torque is changed by providing the flat and sloped portion, as in the conventional single-rotor motor of Patent Literature 2, there is a limit to suppression of combined cogging torque in which inner and outer cogging torques are combined in the dual-rotor motor.                Patent Literature 1: U.S. Pat. No. 6,924,574        Patent Literature 2: Japanese Patent Unexamined Publication No. 2000-209829        